Optical fibers are finding increased applications in environments that are relatively harsh compared to traditional telecommunications operating conditions. For example, in geophysical and geothermal applications (e.g., oil well exploration), optical fibers are deployed in very diverse roles ranging from data logging (requiring high bandwidth fiber) to acting as distributed temperature and pressure sensing elements. The conditions that the optical fiber experiences in such applications can reach temperatures as high as several hundred degrees Centigrade and pressures of several hundred atmospheres in fluid (i.e., gaseous or liquid) environments that contain water, hydrogen, hydrocarbons, sulfides, etc. It is well known that traditional glass optical fibers are very susceptible to both hydrogen and moisture ingress, and therefore the use of optical fibers in such environments generally requires protection from ingressing substances in order to ensure proper function over extended periods of time.
The traditional approach to protecting optical fibers from harsh environments has been the application of an impervious coating on the surface of the glass. Many different coatings have been tried, including metals (see, e.g., Wysocki, “Reduction in Static fatigue of Silica Fibers by Hermetic Jacketing,” Applied Physics Letters 34(1) (January 1979)), ceramics (see, e.g., U.S. Pat. Nos. 4,028,080 and 4,512,629) and carbon (see, e.g., U.S. Pat. No. 4,183,621 and Huff et al., “Amorphous Carbon Hermetically Coated Optical Fibers,” Technical Digest for Optical Fiber Communication Conference, Paper TUG-2 (1988)). Metals tend to form polycrystalline structures in which the grain boundaries act as short-circuit diffusion paths and can themselves become rapidly corroded in harsh environments; additionally, even soft metals such as tin and aluminum tend to induce losses due to micro-bending.
Ceramic coatings, such as silicon nitride or silicon carbide, have been demonstrated to be effective in providing resistance to water at elevated temperatures and pressures (as evidenced by high stress-corrosion parameter values). For these coatings, however, the mean strength generally falls significantly below the value for standard polymer-coated glass fibers, and as is the case with metals, the high moduli of the coating materials tend to give rise to significant micro-bending losses. Hence, neither ceramic nor metal coatings are ideal in terms of combining strength, hermeticity and resistance to bend loss in the same fiber.
Carbon coatings can provide these properties, however, at least at relatively low temperatures (about 100° C. or below). For example, at such temperatures saturation lifetimes with respect to hydrogen ingress are on the order of years; micro-bending is minimal (for relatively small coating thicknesses) and mean strength, while on average probably still below the optimal values obtained for polymer-coated fibers, can be improved with process/roughness control during the deposition of the carbon. At temperatures above about 150° C., hermeticity with respect to hydrogen ingression starts to degrade, and the strong exponential dependence of the diffusion coefficient of H2 through the carbon coating makes it more permeable to H2 at temperatures greater than about 100° C., with saturation being achieved in a matter of days. Carbon-coated optical fibers have been manufactured for some time (see Huff et al., supra), and the permeability of these coatings with respect to hydrogen diffusion has been extensively studied (see, e.g., LeMaire et al., “Hydrogen permeation in optical fibers with hermetic carbon coatings,” Electron Lett. 24:1323-1324 (1988)). The time dependence of the change in attenuation (e.g., at 1.24 μm) due to hydrogen in the glass fiber is given by:Δα1.24(t,T)/{Δα1.24(inf,T)×PH2}=[1−exp {−(t−τi)/τf}]  (1)where Δα1.24(t,T) is the change in attenuation (at 1.24 μm) after the fiber has been exposed to a hydrogen environment for time t at temperature T; Δα1.24(inf,T) is the change in attenuation when equilibrium has been reached between the fiber and the environment; and PH2 is the hydrogen pressure. τi and τf are, respectively, the time constants for initial lag (before any increase in attenuation is observed) and the rate at which the attenuation increases at any given temperature T. Also, for relatively thin coatings, τi is generally much less than τf. For relatively short times, then, equation (1) becomesΔα1.24(t,T)/{Δα1.24(inf,T)×PH2}≈[t/τf]  (2)
A plot of the left side of equation (2) vs. time “t” should yield a line with slope 1/τf. τf and τi describe the permeation characteristics of the carbon coating. Larger values of τf imply a increasingly resistant coatings, and maximizing its magnitude has been the object of much research. Most studies to date that have tried to optimize the value of τf have focused on the carbon deposition conditions, precursor gases, and the like (see, e.g., U.S. Pat. No. 5,000,541 and Aikawa et al., IWCS Proceedings at 374 (1993)).